1. Field of the Invention
This invention relates primarily to a method for producing a screen for papermaking with watermarks, in particular for producing papers of value, such as bank notes, checks, identification documents and other security papers. The invention in addition relates to a mask for use in the aforementioned method, a papermaking screen to be produced by said method and a paper of value produced by means of such a papermaking screen.
2. Description of the Background Art
Papermaking is done in continuous fashion on so-called cylinder paper machines or fourdrinier paper machines, whereby paper pulp is deposited on a moving screen and consolidated to the extent that it can be removed from the screen as wet paper web for further processing such as drying, etc.
In the production of watermarked paper, a distinction is made between two-level watermarks with a strong light-dark effect and multilevel watermarks with many soft transitions between light and dark. To produce two-level watermarks, metal wires or metal moldings (so-called electrotypes) are soldered onto the screen structure to completely close the screen at these places. Thus, paper deposit is hindered in these partial areas of the screen, and thin places form in the paper that appear very light when viewed in transmission. In the production of multilevel watermarks, however, no holes of the screen are closed. Instead, a three-dimensional relief is embossed into the screen so that the paper thickness of the finished paper varies in accordance with the relief and shows soft transitions between light and dark areas in transmission.
WO 99/14425 describes an apparatus for producing absorbent paper and relates in particular to a patterned drying screen on which a paper web is transported or dried. The drying screen comprises not only a two-layer reinforcing structure but also a relief structure of cured photoresist applied thereto.
A method for applying a relief structure to a screen is described in U.S. Pat. No. 4,514,345. A thus produced screen is used for providing an existing paper layer with a three-dimensional pattern, the pattern arising by deflecting the fibers into the relief structure.
EP-A-0 367 520 describes different methods for fastening flexible watermark designs to a screen or a pressing wire. Said designs serve to produce watermarks in the later papermaking.
Unlike two-level watermarks, multilevel watermarks can only be produced on cylinder paper machines since the screens of cylinder paper machines are of rigid form. On fourdrinier paper machines, however, the screen is formed as a moving endless belt and an embossed relief for producing multilevel watermarks would be destroyed by the repeated deflection. But papermaking on cylinder paper machines is time-consuming and cost-intensive, so that embossed, multilevel watermarks are only used in the production of security-relevant papers, such as bank notes or other papers of value.
The production of two-level watermarks, although at first glance more versatile, is also problematic in many respects. Thus, soldering the metal wires or electrotypes onto the screen requires elaborate hand labor by highly skilled specialists and is therefore time-consuming and costly. Further, the firmly soldered parts are subjected to great mechanical stress on fourdrinier paper machines and not infrequently come off the screen, which has a negative effect on production reliability and product quality since the loose parts can collect in the screen and destroy the paper.
Similar problems also occur with cylinder paper machines, in particular if the two-level watermark is combined with an embossed multilevel watermark, for example to emphasize certain details, writing, coats of arms or the like by special lightness in a multilevel watermark.
Above all, the adhesion of electrotypes in the relief-embossed screen areas is critical. In addition, exact positioning of electrotypes in the relief-embossed areas of the screen is difficult. It is accordingly problematic to refasten detached parts at exactly the same position.